Seismic data processing method and system for migration of seismic signals incorporating azimuthal variations in the velocity

ABSTRACT

A method and system for seismic data processing utilizes azimuthal variations in the velocity of seismic signals. The system and method utilizes a plurality of seismic energy sources that are located at known positions at the surface of the earth. The seismic energy sources generate seismic signals that propagate downward into the earth. Some of the seismic signals are reflected and diffracted by various sub-surface layers and are returned to the surface of the earth. The returned seismic signals are received by a plurality of receivers. The method includes the step of determining the distance from an energy source to an image point. A fast travel time of the seismic signal from the energy source to the image point is determined, and a slow travel time of the seismic signal from the energy source to the image point is determined. The azimuth angle between the energy source and the surface location of the image point is calculated. A first travel time of the seismic signal traveling from the energy source to the image point is calculated. A second travel time of the seismic signal traveling from the image point to the seismic receiver is calculated. The total travel time is calculated by adding the first and second travel time. The amplitudes from the recorded signal at the total travel time are phase adjusted and added into the output image at the image point. The foregoing steps are repeated for a plurality of image points beneath the surface of the earth and the total travel time is calculated.

FIELD OF THE INVENTION

The invention relates generally to seismic data processing. Morespecifically, the invention relates to a seismic data migration methodand system that incorporates azimuth variations in the velocity ofpropagation of seismic signals.

BACKGROUND OF THE INVENTION

Seismic surveys are used to evaluate the geometry and properties ofsubsurface rocks. The subsurface geometry and properties often indicatehydrocarbon deposits.

In seismic surveys, seismic energy sources are used to generate seismicsignals at or near the surface of the earth. The seismic signalspropagate downward into the earth and are reflected and diffracted bydiscontinuities in the subsurface. Some of the signals are returned backto the surface where they are detected by seismic sensors.

Seismic sensors are deployed on the surface of the earth. A seismicsensor may be a transducer that converts the seismic signals intoelectrical signals. The electrical signals from each sensor aretransmitted and recorded for processing.

The sensors record the amplitude of the seismic signals arriving at thesurface location of the sensor and also record the round-trip traveltime of the signals from the seismic energy sources on the surface to areflector and back to the surface. A display of the raw recorded signalsdoes not provide a true picture of the reflectors in the subsurface.

The subsurface is a non-uniform medium, which causes spatial variationsin the propagation velocity of the seismic signals, resulting invariations in the direction of propagation of the signals. These effectsof the non-uniform medium on the seismic signals are calledinterferences.

At the boundaries between rock layers and faults, a part of the seismicsignal undergoes reflection. The reflected signals from many reflectorsarrive at the same receiver at the same time, which causes the recordedsignals to appear very mixed. FIG. 1 shows recorded seismic signals thatare not corrected. As will be understood by those skilled in the art, inFIG. 1, the dipping layer reflections and the fault reflection overlayeach other and the subsurface structure is not resolved. As a result,the subsurface structure appears very confusing.

In seismic data processing, a numerical method known as migration isused to focus the recorded seismic signals and to move (i.e., migrate)the reflections in the seismic data to their correct spatial positions.In migrated seismic data, the locations of geological structures such asfaults are more accurately represented, thereby improving seismicinterpretation and mapping. FIG. 2 shows the seismic signals aftermigration is applied to the seismic signals shown in FIG. 1. As will beunderstood by those skilled in the art, the recorded signals have beenmoved (i.e., migrated) to the spatial location of the rock boundariesthat caused the reflections. Consequently, the fault and the reflectinglayers are well focused.

There are many different migration methods. Examples include: frequencydomain, finite difference, and Kirchhoff migration. In general, thesemigration methods involve the back propagation of the seismic signalsrecorded at the surface of the earth to the region where it wasreflected. In Kirchhoff migration, the back propagation is calculated byusing the Kirchhoff integral representation. According to Kirchhoffintegration, the signals recorded at the surface that originated at agiven subsurface image location are summed. In order to compute theKirchhoff integration, the travel times from the subsurface imagelocation to each source and receiver location at the surface arerequired. The computation of the travel times requires a model of theseismic propagation velocity.

In existing methods, the starting seismic velocity model is determinedas an independent processing step before migration. Errors in thevelocity model are determined by examining the output of the migration.The velocity model is updated to correct for the measured errors andmigration is applied to the data using the updated velocity model. Whenthe errors in the velocity model have been reduced to a satisfactorylevel, the final migration is applied.

FIG. 3 shows the field geometry for a seismic signal generated at asingle source, reflected from a single image and recorded at a singlereceiver, i.e., a seismic sensor. An analytical expression for thetravel time of a seismic signal, t, from the source S to the image pointI to the receiver G is given by,

$t = {\sqrt{\left( \frac{t_{0}}{2} \right)^{2} + \frac{{{\overset{\rightarrow}{r}}_{s}}^{2}}{V_{rms}^{2}} + {C{{\overset{\rightarrow}{r}}_{s}}^{4}}} + \sqrt{\left( \frac{t_{0}}{2} \right)^{2} + \frac{{{\overset{\rightarrow}{r}}_{g}}^{2}}{V_{rms}^{2}} + {C{{\overset{\rightarrow}{r}}_{g}}^{4}}}}$${where},{C = {\frac{1}{4}\frac{\mu_{2}^{2} - \mu_{4}}{\left( {t_{0}/2} \right)^{2}\mu_{2}^{4}}}}$${and},{\mu_{j} = {\frac{1}{t_{0}/2}{\sum\limits_{i = 1}^{N}{v_{i}^{j}\Delta \; t_{i}}}}}$

and, v_(i) is the interval velocity of the seismic signal (i.e., seismicwave) of each earth layer from the surface to the depth of the imagepoint and V_(rms) is the Root Mean Square (RMS) velocity of the seismicsignal from the surface to the image point.

$V_{rms}^{2} = {\frac{1}{t_{0}}{\sum\limits_{i = 1}^{N}{V_{i}^{j}\Delta \; t_{i}}}}$

This analytical equation for the computation of travel time of theseismic signal is a fourth order approximation. The fourth orderequation provides more accurate travel times than the second orderequation when the earth velocity model has a gradient increasing withdepth.

An alternate method for computing the travel time of the seismic signalutilizes ray tracing. According to the ray tracing method, a table orgrid of the subsurface is populated with a value corresponding to theinterval velocity of the seismic signal at each point in the subsurface.As will be understood by those skilled in the art, a seismic signal isgenerated at the image location, and the signal is propagated throughthe grid using finite differences, eikonal equation solutions, or directray tracing using Snell's law. The transit time of the signal from theimage location to each of the source and receiver locations is measuredand used in the migration. Ray trace solutions account for the curvatureof rays in the earth caused by the vertical velocity gradient, whichproduces superior quality images. Pre-computing the travel time tablesprovides for a significant improvement in efficiency. The travel timetables are computed and stored for later use in the imaging.Pre-computing and storing the travel time tables also allows for theinclusion of azimuth variations in the velocity.

In existing time migration methods, it is assumed that there are noazimuthal variations of the velocity. However, for geologic regimesunder tectonic stress, it is well documented that azimuthal variationsof the velocity exists. Fractures resulting from stress fields causeadditional azimuthal variations of the velocity. In existing methods,the analysis for azimuthal variations in velocity is carried out beforeimaging. The recorded seismic signals are gathered in azimuth corridorsaccording to the azimuth direction between the source location and thereceiver location and then imaged using isotropic imaging. A differentvelocity model is used for each of the azimuth corridors. For recordedsignals that have not been focused using migration, the azimuthalanalysis is compromised by the mixing of signals from multiplereflection locations. In addition, the gathering of the data accordingto the azimuth between the surface locations of the source and receiverignores the real propagation direction of the signal from the source tothe reflection point and from the reflection point to the receiver.

Accordingly, there is a need for a seismic data processing method andsystem that incorporates azimuthal variations of velocity in migrationof the seismic signals for improved imaging of geological structures.

SUMMARY OF THE INVENTION

The invention is a method and system for seismic data processingutilizing the azimuthal variations in the velocity of seismic signals.In one embodiment, the system and method utilizes a plurality of seismicenergy sources located at known positions at the surface of the earth.The seismic energy sources generate seismic signals that propagatedownward into the earth. Some of the seismic signals are reflected anddiffracted by various sub-surface layers and are returned to the surfaceof the earth. The returned seismic signals are received by a pluralityof receivers.

The method includes the step of determining the distance from an energysource to an image point. A fast travel time of the seismic signal fromthe energy source to the image point is determined, and a slow traveltime of the seismic signal from the energy source to the image point isdetermined.

The azimuth angle between the energy source and the surface location ofthe image point is calculated. A first travel time of the seismic signaltraveling from the energy source to the image point is calculated. Asecond travel time of the seismic signal traveling from the image pointto the seismic receiver is calculated. The total travel time iscalculated by adding the first and second travel time. The amplitudesfrom the recorded signal at the total travel time are phase adjusted andadded into the output image at the image point. The foregoing steps arerepeated for a plurality of image points beneath the surface of theearth and the total travel time is calculated.

The fast travel time is calculated using the distance from the energysource to the surface location of the image point, the travel time fromthe surface location of the image point to the image point, and a fastvelocity table. The slow travel time is calculated using the distancefrom the surface location of the image point to the seismic receiver,the travel time from the surface location of the image point to theimage point, and a slow velocity table. The azimuthal angle between theenergy source and the surface location of the image point is calculatedusing the respective coordinates.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures and in which:

FIG. 1 shows an image of a geological structure before migration,wherein the dipping bed reflections and the fault reflection overlayeach other and cause the subsurface structure to appear very confusing.

FIG. 2 shows an image of a geological structure after migration of theunderlying data of the seismic signals illustrated in FIG. 1, whereinthe recorded signals have been moved to the locations of the originalreflections.

FIG. 3 shows the field geometry for a seismic signal generated at asingle source, reflected from a single image point and recorded at asingle receiver.

FIG. 4 shows the variation of the velocity of the seismic signal as afunction of theta, represented by an elliptical function.

FIG. 5 shows a travel time table.

FIG. 6 shows, in map view, the distances and azimuths between thesource, the image point location, and the receiver.

FIG. 7 is a flow diagram of the steps for calculating the travel time tfrom the source S to the image point I to the receiver G.

FIG. 8 is a computer system configured to carry out the steps inaccordance with one embodiment of the invention.

FIG. 9 compares the signal quality when isotropic migration methods areused and when azimuthal velocity variations are accounted for in themigration method.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the seismic data processing method and system relates toKirchhoff pre-stack time migration. As will be apparent to those skilledin the art, the term pre-stack refers to a recorded signal whereby thesource location and receiver location geometries have been retained. Aseismic data processing method commonly referred to as “stacking” can beapplied to recorded signals. Stacking modifies each of the recordedsignals and then sums it with adjacent signals to produce signals wherethe source and receiver are co-located. As will be understood by thoseskilled in the art, pre-stack migration is applied to seismic data inwhich the source and the receiver for each input trace are at spatiallyseparated surface locations (i.e., not co-located). Pre-stack migrationprovides an improved quality image because the signals are not stretchedand mixed before the migration process is applied.

The seismic data processing method and system provides for improvedimaging of geological structures by incorporating azimuthal variationsof the velocity of the seismic signals. In one embodiment, a Kirchhoffpre-stack time migration incorporates variations in the velocity of theseismic signals as a function of azimuth.

In order to incorporate the azimuthal variations in the velocity ofpropagation for seismic signals into the Kirchhoff migration method, itis necessary to measure the parameters for the azimuthal variations.Three parameters are required: velocity, azimuth direction, and azimuthmagnitude.

The velocity is determined in the following manner. The starting seismicvelocity model is determined before migration as an independentprocessing step. A first pass of migration is applied at a large numberof locations and errors in the velocity model are determined byexamining the output of the migration. The velocity model is updated tocorrect for the measured errors and migration is applied to the datausing the updated velocity model. When the errors in the velocity modelhave been reduced to a satisfactory level, the velocity model isfinalized.

The azimuthal magnitude and direction parameters can be derived usingthe unmigrated signals. A large number of signals are collected togetherthat are in the same surface location. These signals are combined andthen sorted by the azimuth direction between the source and receiversurface locations. By observing the change in travel time as a functionof azimuth for selected reflection events, the values of the magnitudeand direction parameters can be determined. This analysis is repeated ata large number of surface locations and for a number of reflections ateach location. A volume of azimuthal magnitude and direction parametersis created for use in azimuthal migration.

The application of azimuthal migration assumes that the azimuthalvariations of velocity fit an elliptical model. The direction of thefast velocity and the slow velocity are assumed to be perpendicular toeach other. Because travel times are inversely proportional to velocity,the major axis of the ellipse may represent the slow velocity and theminor axis of the ellipse may represent the fast velocity. An example ofthis ellipse is shown in FIG. 4.

Isotropic migration using a stored travel time table approach requiresthat one travel time table be stored for each output image surfacelocation. For azimuthal migration, two travel time tables must bestored. The travel time computation uses the velocity and azimuthalmagnitude values to compute the two travel time tables for a selectedsurface location.

The travel time tables are computed from two velocity functions. The twovelocity functions are computed by multiplying the input velocityfunction by the percent magnitude value and then adding the result tothe input velocity and subtracting the result from the input velocity.Using the two computed velocity functions, two travel time tables arecomputed. These two travel time tables are stored for use in themigration algorithm. FIG. 5 shows an example of a travel time table.

FIG. 3 shows the source location S, the image point I, the surfacelocation O of the image point I, and the receiver location G. The traveltime t from the source S to the image point I to the receiver G iscomputed using the steps Illustrated in FIG. 7. FIG. 6 shows a map viewof the surface locations for the source, the image point, and thereceiver. The corresponding distances and azimuths are indicated in FIG.6.

The two travel time tables and the azimuth direction of the fastvelocity are used as input for each image point. Referring now to FIG.7, in step 704, for each image point, the distance along the surfacefrom the image point to the source location is computed from the inputlocation data (S-I Distance in FIG. 6). In steps 708 and 712, two traveltimes are computed, one from each travel time table. In step 716, theazimuth direction from the image point to the surface location of thesource is computed (I to S Az in FIG. 6). In step 720, using the traveltime from the slow velocity table as the major axis of an ellipse andthe travel time from the fast velocity table as the minor axis of theellipse, the desired travel time for the image point to the surfacelocation of the source is computed. The computation is carried out usingan elliptical formula with the two travel times from the tables as theaxes of the ellipse. The value of the ellipse for the azimuthaldirection from the image point to the source location is the desiredtravel time for the image point to the source location. In step 724, theprocess is repeated for the receiver location using R-I Distance and Ito R Az shown in FIG. 6. Note that the same travel time tables are usedfor both the source to image point calculation and the receiver to imagepoint calculation. In step 728, the total travel time for the signalpropagation from the source to the image point to the receiver iscalculated.

The amplitude data from the input signal (i.e., seismic signal from thesource) at the calculated travel time is summed into the output image.In step 732, steps 704 through 728 are repeated for each input signaland for each image point in the 3D volume.

Utilizing the azimuthal information in computing the travel times forthe migration algorithm yields travel times that can be significantlydifferent from the travel times computed for isotropic migration asillustrated in FIGS. 9A and 9B. A collection of signals after migrationusing travel times for isotropic migration is illustrated in FIG. 9A.FIG. 9B illustrates a collection of signals after migration using traveltimes for azimuthal migration. As shown in FIG. 9B, the reflection eventis significantly improved when azimuthal travel times are used in themigration. It will be apparent to those skilled in the art how toutilize the signals incorporating the azimuthal variations in velocityin accordance with the present invention to evaluate the geometry andproperties of subsurface rocks and other seismic data processing tasks.

In another embodiment of the invention, the travel times for azimuthalmigration are determined by manipulating the fast and slow velocitiesdirectly. First, the fast velocity and slow velocities for each surfacelocation are computed and stored. Next, for an image point to a sourceor receiver location, the velocity for the azimuth direction iscalculated using an elliptical model and the fast and slow velocities.The calculated velocity is used in an analytical equation to obtain thedesired travel time for migration. In this embodiment, the travel timesare not pre-computed or stored and the elliptical model is applied tothe fast and slow velocities rather than the travel times.

FIG. 8 is a system 800 configured to carry out the steps in accordancewith one embodiment of the invention. The system comprises a computer804 having a processor 808 and a memory 812. A program code 816 forcarrying out the steps (e.g., steps shown in FIG. 7) of the inventionresides in the memory 812. An input device 820 is configured to receiveseismic data and adapted to provide the seismic data to the processor.The processor 808 is configured to execute the program code 816, thuscarrying out the steps in accordance with one embodiment of theinvention. The system 800 includes an output device such as a monitor ora printer to output the results.

The program code for carrying out various steps of the invention can bewritten in computing language such as C, C++, assembly language, etc.The program code can be stored in any storage medium such as a harddrive, a CD ROM or any other memory device.

It will be understood by those skilled in the art, that the travel timescan be calculated using ray trace methods or any other alternativemethod.

While the structures, apparatus and methods of this invention have beendescribed in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thestructures, apparatus and/or methods and in the steps or in the sequenceof steps of the method described herein without departing from theconcept, spirit and scope of the invention. For example, variousdistances, travel times, and azimuthal angles discussed in the foregoingcan be calculated using one or more methods that will be apparent tothose skilled in the art. All such substitutes and modificationsapparent to those skilled in the art are deemed to be within the spirit,scope and concept of the invention as defined by the appended claims.

1. A method of seismic data processing for migration that incorporatesvariations in velocity with azimuth, the method utilizing a plurality ofseismic energy sources and seismic receivers located at known positionsat the surface of the earth and a plurality of image points located atpredetermined depths beneath the surface, each seismic energy sourcegenerating one or more seismic signals that propagate into the earth andreturn to the surface where the seismic signals are received by theseismic receivers, the method comprising the steps of: determining thedistance from the energy source to the surface location of the imagepoint; determining the fast travel time of the seismic signal from theenergy source to the image point; determining the slow travel time ofthe seismic signal from the energy source to the image point;determining the azimuth angle between the energy source and the surfacelocation of the image point; determining a first azimuth dependenttravel time for the seismic signal traveling from the energy source tothe image point using the fast travel time, the slow travel time, theazimuth angle of the fast velocity direction, and the azimuth angle ofthe energy source location to the image point location on the surface;determining the distance from the receiver to the surface location ofthe image point; determining the fast travel time of the seismic signalfrom the receiver to the image point; determining the slow travel timeof the seismic signal from the receiver to the image point; determiningthe azimuth angle between the receiver and the surface location of theimage point; and determining a second azimuth dependent travel time forthe seismic signal traveling from the receiver to the image point usingthe fast travel time, the slow travel time, the azimuth angle of thefast velocity direction, and the azimuth angle of the receiver locationto the image point on the surface. 2-10. (canceled)
 11. A system forseismic data processing by incorporating the azimuthal variations of thevelocity of seismic signals, comprising: a computer comprising; aprocessor; a memory coupled to the processor; an input device coupled tothe processor and adapted to receive seismic data and to provide theseismic data to the processor, a program code stored in the memory andexecutable by the processor to process the seismic data by performingthe following steps: determining the distance from an energy source toan image point; determining the fast travel time of the seismic signalfrom an energy source to an image point; determining the slow traveltime of the seismic signal from the energy source to the image point;determining the azimuth angle between the energy source and the surfacelocation of the image point, determining a first azimuth dependenttravel time of the seismic signal traveling from the energy source tothe image point; determining the distance from a receiver to an imagepoint; determining the fast travel time of the seismic signal from areceiver to an image point; determining the slow travel time of theseismic signal from the receiver to the image point; determining theazimuth angle between the receiver and the surface location of the imagepoint, determining a second azimuth dependent travel time of the seismicsignal traveling from the image point to the receiver; calculating thetotal travel time by adding the first and second azimuth dependenttravel times; summing the amplitudes from the input signals at the totaltravel time, wherein the signals are phase adjusted according theKirchhoff imaging criteria and then summed into the output image at theimage point; and repeating the foregoing steps for a plurality of imagepoints beneath the surface of the earth and calculating the total traveltime for the plurality of image points.
 12. The system of claim 11,wherein the fast travel time is calculated using the distance from theenergy source to the surface location of the image point, the traveltime from the surface location of the image point to the image point,and a fast velocity table.
 13. The system of claim 11, wherein the slowtravel time is calculated using the distance from the energy source tothe surface location of the image point, the travel time from thesurface location of the image point to the image point, and a slowvelocity table.
 14. The system of claim 11, wherein the azimuthal anglebetween the energy source and the surface location of the image pointand the azimuthal angle between the energy source and the surfacelocation of the image point are calculated using the respectivecoordinates.
 15. (canceled)
 16. A method of seismic data processing,comprising the steps of: determining the azimuthal variations in thevelocity of seismic signals, the seismic signals propagating from one ormore seismic energy sources to one or more image points and to one ormore seismic receivers; determining the travel times of the seismicsignals using the azimuthal variations in the velocity of the seismicsignals, and using the travel times in migration methods.
 17. The methodof claim 16, wherein the travel times are determined using an ellipticalfunction when provided a fast velocity and a slow velocity.
 18. A methodof seismic data processing, comprising the steps of: determining theazimuthal variations in the velocity of seismic signals, the seismicsignals propagating from one or more seismic energy sources to one ormore image points and to one or more seismic receivers; calculating thetravel times of the seismic signals using the azimuthal variations inthe velocity of the seismic signals; using the travel times in migrationmethods and generating image of the seismic signals using the traveltimes.